ive just done this with mine a month ago. What you don't want to do is plop the new guinea pig into your existing guinea pig's cage. Never try to introduce guinea pigs in one of their cages. Be prepared. This may happen in one afternoon or it may take months!
Get a large bath towel or two.
Put the towels on the couch (neutral, unfamiliar territory).
Spread them out over the middle of the couch.
Each person holding a pig, sits at opposite ends of the couch.
Make sure the towels are between you and there is a good amount of space (it helps to have a big couch!). Floor works, too. Keep kids quiet and out of reach. No other distractions.
Let the pigs find each other on their own time.
You may need to nudge them in the right direction. Have another towel handy to toss on the pigs if you need to separate them.
More often than not, pet owners want to break up guinea pigs exhibiting normal dominant behavior.